Secretory and endocytic processes in several cell types are currently under investigation. The rat basophilic leukemia cell line (RBL-2H3), and other cultured cells are being used to study various aspects of endocytic and secretory processes. Emphasis is placed on morphological, cytochemical and biochemical characterization of these processes in the cultured cells. Events involved in receptor activation, signal transduction and endocytic mechanisms are under investigation. The lysosomal system and its role in endocytic and secretory pathways is also under study.